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Celestial Navigation – On-board training with superyacht ‘Kamalaya’

Over Christmas & New Year 2014/2015 Alan Denham (RYA Yachtmaster Ocean Instructor) was invited aboard the 55 metre (180’) Amels Superyacht. The vessel was in transit across the North Atlantic from Lanzarote to St Marteens in the Caribbean. The plan was to help the Captain to improve his Astro-navigation skills and to train 3 of the deck officers/senior crew so they would be well on their way to achieve their RYA Yachtmaster Ocean Certificate.

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Category: Navigation

The Sextant and its Corrections

The Sextant is a very accurate and technical device for doing something very simple - measuring angles. You could use a protractor and a piece of string, though accuracy would not be very good!

The Sextant is designed to measure angles very precisely, to an accuracy of Degrees, Minutes and 10th’s of minutes (we call 1/10th of a Nautical Mile a Cable). We normally use the Sextant to measure the angle between the horizon and a vertical object.

In Coastal Navigation we could measure the angle between our horizon and the top of a lighthouse. This can tell us how far we are from the Lighthouse. For those interested, it’s the Tangent Rule, though you don’t need to know this as a table called ‘Vertical Sextant Angles’ is readilly available in many marine publications such as ‘Reeds’ Nautical Almanac.

The MCA in their infinite wisdom have decided to use the traditional method of using 12 hour clocks on their vessels. This means that 2 clocks with hands are carried. One is used to measure SHIPS TIME (Local time), the other, the CHRONOMETER or DECK WATCH is used to measure UT.

The problem with this is that your CHRONOMETER (with its hands), can show 2 potential times,